Rotational molding is a process for the fabrication of parts from powdered thermoplastic material. A split mold is utilized, the two halves of which are clamped together after the proper amount of powdered thermoplastic material has been inserted therein. The mold then is heated while being simultaneously rotated about two axes at right angles to each other. During the heating and rotation cycle the hot thermoplastic material fuses together forming a homogeneous layer contoured to the surface of the mold. The duration of and temperature during the heating cycle as well as the frequency and characteristics of the rotation about each axis are important variables in determining the quality of the molded plastic part and are usually empirically determined.
Rotational molding devices traditionally consist of a heating chamber, a cooling chamber, a holding mechanism for the molds, and a drive mechanism for rotating the molds simultaneously in orthogonal axes. The apparatus disclosed herein utilizes the functional elements above but incorporates unique structure for rotating the molds and translating the molds into desired positions. With this unique structure, a process of rotational molding is disclosed which was heretofore not possible.
Prior art rotational molding apparatus traditionally consists of machinery having separate arms extending therefrom and spaced 120.degree. apart. Also included is a heating chamber, a cooling chamber and a mold rework station. Drive motors rotate the arms, which are traditionally horizontally disposed, so that one arm is positioned within each of the chambers and the mold rework station at a time. Spiders carrying mold halves are attached at the exterior of the arms and a power driving mechanism is included to rotate the spiders and thus the molds attached thereto simultaneously about a major axis aligned with the longitudinal axis of the arm and a minor axis disposed orthogonally to the major axis. In many instances all or part of the power drive mechanism is located in such a position that it may be subjected to the adverse environmental conditions extant in the oven and the cooling chambers. Alternatively the orthogonal rotation is obtained in some prior art devices by rocking the spiders in one direction while rotating in the orthogonally disposed direction or axis.
It will become apparent to those skilled in the art that sufficient clearance must be provided both above and below the longitudinal axis of the arms supporting the spiders to provide for proper dual axis rotation of the molds. As a result, the arms are usually raised a substantial distance above the ground or floor level where the machinery is installed or, alternatively excavation is required at the stations where the rotation occurs. In either event added cost for installation of the equipment results and typically the positioning of the mold rework station is such that mold rework becomes a complicated, time consuming and thus expensive operation.